[AMPS] two questions, TWO problems

[w8jitom@postoffice.worldnet.att.net]

> From:          "Ian White, G3SEK" <G3SEK@ifwtech.demon.co.uk>
> Subject:       Re: [AMPS] two questions
> Date:          Sat, 6 Sep 97 07:43:25 +0000

> A number of VHF mountain-toppers are using modified computer switch-mode
> supplies for 6V heaters, to avoid problems with generator sags and
> surges. Apparently the fully-enclosed and filtered units work well, but
> I'm sure an open-frame or unfiltered switcher would be a disaster on
> receive. Also, this is information relates to VHF - suppression of
> switching transients may not be so good at HF.
> 
> Bottom line: it's worth a try.

But be careful!

The LAST thing you want at turn on is a stiff voltage regulated 
supply! It would be inrush city for the filament.

Also, many supplies have fold-back current limiting. Those supplies 
will often NEVER come out of limiting and run a filament!

Finally, if the supply does come out of limiting slowly, the filament 
warm up time is increased. If you use a MOX cathode tube you could 
strip the cathode from insufficient warm up time with a fold 
back current limited supply, IF it ever comes up at all. 

By the way, the largest failures in amateur PA tubes are NOT related 
to excessive filament voltage . The most common 
non-tube-manufacturing related defects are gas or dissipation 
related damage, not low emission or open filaments.

 

73, Tom W8JI

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